a polarity tester might come in handy, but you don't need it.
when swapping the mags:
the narrow ends and the top and bottom should repel, the sides should attract.
if you mess this up in a humbucker it's not usually a big deal, as there's usually only 1 mag in there; if you do get it backwards, it and your other p/up will be out of phase together but only when they're both at the same volume; the same effect as using green for hot instead of black (and using black as ground). IMO an LP is a good guitar to do this with, as you have a volume control for each p/up.
i don't have a hair-dryer and YMMV, but i heat up the p/up's baseplate at each end and in the middle with the solder iron to melt the wax again, push it all together and tighten the bobbin screws. you can usually tell when it's heated up (10 seconds max) as the baseplate gets shiny
